An assembly process can usually be decomposed into a sequence of tasks each designed to bring pairs of objects together and join them. The process is simplified in several ways if some form of part-to-part homing is used in bringing parts together. A military example of assembly by homing is that of a cruise missile assembling itself onto the center of the unfortunate target. The homing information comes from comparing on board sensor data with stored maps. The alignment of a photo-detector with a light carrying fiber is an industrial example where direct part-to-part homing may be used. Biological examples of sensor homing abound. Direct homing implies that one part senses alignment information from the other.
Lacking a sense of homing, industrial assembly often uses some form of control by schedule during the bringing together or closure phase of the operation.
Consider, for example, the so called "pick and place" assembly procedure. The separated objects are brought together by picking up one object and placing it onto the other. Before the objects can be placed, relative locations and orientations, the configuration of the mating surfaces and the assembly mechanisms need to be established. The configuration is determined either by sensing, using for example machine vision, or more often by mechanical means such as from the geometric constraints imposed by parts feeders, jigs and holding fixtures. In the picking phase one part is engaged by a gripping or holding tool of the mechanism, temporarily assembling it to the tool. The parts are brought together into an aligned condition in a third closure phase using a scheduled motion that is in accordance with estimates of the beginning configuration and the desired final configuration. The parts are joined in a fourth phase. Joining may require a complicated mechanical procedure such as screw fastening or, as is the case in this example, a simple release of the moved part. Of course, there are many possible ways to assemble objects. This description is nevertheless quite typical and serves to illustrate the open-loop aspect of presently used control methods. We will not consider the transport and joining tasks. Further examples and details of automatic assembly processes can be found in the book by G. Boothroyd, Corrando Poli and Laurence E. Murch, "Automatic Assembly", Marcel Dekker, New York, (1982).
Let us now return to the closure phase in more detail. The scheduled motion is with respect to the final alignment of parts an open-loop procedure. Only the initial configuration is estimated, no direct or continuos measurement of the relationships of the mating surfaces is available. Many arguments that support the need for high precision and absolute accuracy robots and assembly machines are linked to assembly methods that have restricted knowledge of the configuration of the parts during closure. Parts mating accuracy is then ultimately limited by a combination of uncertainties in the initial configuration estimate, gripping and motion. An attractive alternative approach is to arrange for homing towards the aligned condition. A suitably located machine vision camera can in principle provide such information indirectly while observing the mating surfaces of both parts as they are brought together. However, these measurements are most important during the final closure phase when the parts themselves may obscure the measurement and the assembly must proceed from that point forward using a scheduled motion.
We have recognized that there is great need in this and other phases of the automatic assembly art to minimize reliance open-loop control by providing a direct and continuous measurement of the configuration of the mating surfaces so that closed-loop control can be used throughout critical portions of assembly.
The techniques for closed-loop feedback control are well known in the control system art. The basis of the techniques are described by Benjamin C. Kuo, "Automatic Control Systems," third edition, Prentice Hall, (1975), pp. 1-15. Our description concentrates on the integration of measurement with such techniques.